FIG. 1 is a view illustrating delays of signals entering at an angle of θ via array antennas of a radio frequency (RF) or millimeter wave transception system. The time delay between adjacent antennas in FIG. 1 is “d*sin θ.”
When it is assumed that there is no loss caused by wave propagation between antennas, a time delay between the first antenna and the (N+1)-th antenna is “N*d*sin θ.”
Accordingly, in order to transmit or receive radio signals in a desired direction in the system illustrated in FIG. 1, a controller for delaying time as much as “N*d*sin θ” is required.
FIG. 2 is a circuit diagram of a delay device using an optical delay. The illustrated delay device has an advantage of making a delay exactly without a loss, but requires an E/O converter for converting an electric signal into an optical signal, and an O/E converter for converting an optical signal into an electric signal.
Accordingly, there are problems that the delay device has a large volume, and the number of E/O converters and O/E converters geometrically increases and the cost increases as electric signals become faster.
FIGS. 3A and 3B are views illustrating a circuit of a millimeter wave delay device using an electronic time delay (ETD). The time delay may be adjusted by an inductor and a capacitor, and a switch is disposed at each node to adjust the time delay.
This technique has advantages in terms of size and speed, but cannot be applied to a large area array system requiring much delay time due to a problem that a loss and a size increase as delay time increases.
FIG. 4 is a view illustrating an array antenna system using a phase adjuster. A time delay (Td) is converted into e−jwTd by Fourier transform, and the time delay may be equivalent to a phase delay. That is, since even a long TD may be easily expressed by a phase delay, a large area array can be easily configured when the phase adjuster controllable as shown in FIG. 4 is selected as a structure for the array system.
However, in a phase conversion array system configured by modeling a time delay as a phase delay, there is a limit to transception communication and a radar signal bandwidth. This is because a phase delay and a time delay maintain an equivalent relation therebetween only in a narrow band frequency. Recent communication systems desire to support high-speed communication services using broadband frequencies, and a problem may arise in that case.
FIGS. 5 and 6 illustrate this problem. FIG. 5 is a view illustrating gain waveforms when broadband signals reaching 9-11 GHz are transmitted at an angle of 20 degrees by using a real time delay device. As shown in FIG. 5, it can be seen that beamforming is completely performed over all bands including 9-11 GHz.
However, when the same system is implemented by using a phase adjuster as shown in FIG. 6, beamforming of 10 GHz is performed at an angle of 20 degrees, but beamforming of 9 GHz is performed at an angle of 18 degrees, and beamforming of 11 GHz is performed at an angle of 22 degrees. Therefore, there is a variation in transmission output at the desired angle of 20 degrees according to frequencies. This is called a beam squint phenomenon, which is the most serious problem that occurs in transmitting broadband signals using a phase adjuster.